The invention relates to a method of rendering a hydrophilic, inorganic surface of a mould suitable for the release of synthetic resin products.
Such a method is described in, for example, U.S. Pat. No. 4,251,277. In said Specification the surface of the mould is treated with a polymer composition which contains an organopolysiloxane and a thiofunctional polysiloxane liquid. The polymer composition is a clear liquid whose kinematic viscosity is, for example, of from 75 to 100 cS (75.10.sup.-6 to 100.10.sup.-6 m.sup.2 /s). As is shown in the examples of said United States Patent Specification, the release agent is applied in relatively thick layers to a metal substrate, such as a metal board. Preferably, a layer thickness of from 0.125 mm to 2.5 mm is applied. Said United States Patent Specification states in column 7, line 61 and following that very small layer thicknesses of a few .ANG.ngstrom units may also be applied. However, this seems to be a misconception. The molecular dimensions of the substances used are much larger than a few .ANG.ngstroms. Moreover, the high viscosity is not suitable for obtaining very thin layers. This means that the known solvent can not be used with moulds having a fine surface structure because the structure is masked completely by the rather thick layer of the solvent. The layer also exhibits undesired variations in layer thickness. As with many other known polysiloxane solvents, the use of this known solvent in addition causes the metal surface of the substrate to change into an SiO surface. Such an SiO surface exhibits polarity, to some degree, which causes synthetic resins such as the somewhat polar synthetic resins, for example polyacrylates, polymethacrylates, polycarbonates etc., to exhibit a certain interaction with this surface. It is to be noted that the synthetic resins can be readily released from such a modified surface. However, due to the aforesaid interaction some synthetic resin material will remain in the long run on the surface of the mould. This will cause problems in reproduction processes where very many synthetic resin copies must be made of one mould or where the surface of the mould has a fine structure or texture, or a very smooth surface. The residual synthetic resin material, no matter how little, will veil the texture of the surface of the mould thereby rendering the production of qualitatively good copies impossible. Thus, an early replacement of the mould is necessary.
According to U.S. Pat. No. 4,263,350 the surface of a glass mould is sprinkled, in a humid atmosphere having a water content of at least 55%, with a liquid mixture of dimethyldichlorosilane and trimethylchlorosilane. In the humid atmosphere a dense mist develops in the reaction chamber and an oily film of a polysiloxane is formed, due to hydrolysis of the silane compounds and polymerization of the hydrolized silane compound caused by a condensation reaction. This process and the mould produced exhibit the same disadvantages as the mould and the method of the above-described U.S. Pat. No. 4,251,277. Moreover, the polymerization reaction is an uncontrolled process which leads to local variations in the thickness and composition of the oily film obtained.